Definition of gregorian calendar Gregorian calendar
Gre•go•ri•an cal•en•dar
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What does gregorian calendar mean?
OmegaWiki DictionaryOmegaWiki Dictionary Ω
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gregorian calendar
The system of dates used by most of the world, decreed by and named after, Pope Gregory XIII in 1582.
Wikipedia 
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The Gregorian calendar is the calendar that is used throughout most of the Western world. It began to be used from 1582. It replaced the previous Julian calendar because the Julian Calendar had an error: it added a leap year (with an extra day every four years) with no exceptions. The length of the Julian year was exactly 365.25 days, but the actual time it takes for the Earth to go around the Sun once is closer to 365.2425 days. This difference is just over ten minutes each year.
This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional March 21 as the centuries went by. By the 1500s, it was starting around March 11, ten days 'too early' according to the calendar. So what they did was to move the calendar forward ten days in 1582, and at the same time to make sure it didn't happen again. To do this, they made an exception to the previous 'leap year rule' (add February 29 every four years). There would be no February 29 for every year that ends in 00 - unless it could be divided by 400. So the year 2000 was a leap year anyway, because it can be divided by 400, but 2100, 2200, and 2300 will be common years, with no February 29.
It was first suggested by the Neapolitan doctor Aloysius Lilius, and was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582.
Adoption.
Not every country started using the new calendar stra
